1. Mark G. Baxter
Involvement of Medial Temporal Lobe Structures in Memory and Perception

2. Stuctures Involved hippocampus entorhinal cortex perirhinal cortex
parahippocampal cortex
1 – visual, auditory and somatic information is synthesized in association cortices
2 – information is then conveyed through the perirhinal and parahippocampal cortices
3 – information then passes to the entorhinal cortex -> dentate gyrus -> hippocampus -> subiculum -> entorhinal cortex
**entorhinal cortex = major input and output of the hippocampus
-> damage here = a) severe impairment
b) affects all senory information

3. Medial Temporal Lobe Memory System?
MTL =
MTL = memory +
MEMORY
PERCEPTION
MTL = memory -> storage and recall of facts and events
MTL = memory & perception -> suggested approximately 15 yrs. Ago
-> the “perceptual – mneumonic” hypothesis

4. Evidence for Perception
Eacott, Gaffan & Murray (1994)
DMS and DNMS paradigms
rhinal cortex (entorhinal and perirhinal) lesion
spared visual recognition memory
impairment with little to no time delay between sample and choice test
rhinal cortex not required for remembering familiar objects
Eacott, Gaffan & Murray
first to propose that the rhinal cortex may be involved in visual identification and perceptual function
similar function as the laterally adjacent TE
DMS/DNMS (visual recognition MEMORY tasks)
visual objects are presented first as “sample” objects
then choice test objects are presented (sample vs. foil)
monkey is rewarded for choosing a previously shown object (DMS) or rewarded for choosing the foil (DNMS)
** DMS = delayed matching-to-sample, DMNS = delayed nonmatching-to-sample
What they found:
Spared visual recognition memory
- if perirhinal cortex does purely memory, why could this memory task still be accomplished with a reduced sample (to be remembered)
of two?
Impaired performance with little to no time delay
- memory demands are absent in this task and so there should be no impairment
- there was impairment however = perceptual impairment
Rhinal cortex not required for remembering familiar objects
- lesion in rhinal cortex should impair memory of familiar objects but it didn’t
- supports perceptual function

5. Evidence for Perception - Objects
Buckley and Gaffan (1997 & 1998)
perirhinal cortex lesions
impaired discrimination learning
perceptually identify objects not just remember them
Buckley et al. (2001)
impaired oddity discriminations
taxes OBJECT discriminations rather than discriminations in general
Buckley and Gaffan (1997, 1998)
after ablation animals could not learn to discriminate between:
1) large sets of objects
2) 2D and 3D views of an object
New views of familiar objects
The same objects in new scenes
Buckley et al. (2001)
after ablation animals were impaired on choosing which object “was the odd one out”
-> different viewpoints of objects and faces
-> degraded object images
-> visual scenes
……all of which = complex object discriminations!!

6. Evidence for Perception - Features
Barense et al. (2005)
Bussey, Saksida, Murray (2002)
AB+, CD+, EF-, GH-
vs.
AB+, CD+, AD-, BC-
AB+, CD+, BC-, AD- vs. AB+, CD+, EF-, GH-
hard easy
processed in
perirhinal
-> in either set 2 conditions are rewarded (+) and two conditions are not (-)
-> because of that these two paradigms place equal demands on memory
-> if the perirhinal cortex is only visual learning and memory, we would expect a lesion here to produce impairments
in minimum (on right above), intermediate and maximum (on right above) feature ambiguity conditions
-> feature ambiguity – eg. in hard set, B can be either rewarded or not based on what other letter appears with it
Bussey, Sakisida & Murray (2002)
Perirhinal cortex is thought to be an extension of ventral stream processing
-> processes conjunctions of visual features
-> therefore discriminations problems that can only be solved based on a combo of features (on right at top) should
require the perirhinal cortex
Perirhinal cortex lesions in monkeys
-> each photo = a feature
-> perirhinal lesion = greatest impairment with maximum feature ambiguity, not equal impairment across a ambiguity levels
Barense et al. (2005)
- same pattern of impairment as previous study
- feature here = body or leg type
-> minimal ambiguity = each body type and leg type always or never rewarded
-> maximum ambiguity = each body type and leg type are in always rewarded AND never rewarded
- results: MTL (including perirhinal cortex) = impairment

7. Evidence for Percetion - Scenes
Lee et al. (2005)
Previous studies all saw impairment with perirhinal lesions
also evidence that the hippocampus may also have perceptual function
Lee et al. (2005)
human subjects with hippocampal selective lesions
impaired at oddity discriminations among scenes

8. Criticisms against Percetion - Anatomical
damage outside MTL in patient studies
however…
performance similarities between human and monkey models
little performance variation despite difference in extent of lesion damage
fMRI results in neurologically intact humans
performance similarities between humans and monkeys
monkeys have discrete lesions so performance similarities on nearly identical tasks between humans and monkeys is compelling (Bussey vs. Barense)
little performance variation despite differences in extent of human lesions
if lateral temporal damage is what impairs perception then subjects with this type of damage should be much worse at a discrimination task than those with MTL lesions which isn’t the case
fMRI results in neurologically intact humans
perirhinal cortex activation in:
-> oddity judgements
-> feature ambiguity

9. Criticisms against Perception - Cognitive
impairments are memory and learning, rather than, perceptually related
feature-ambiguity and oddity discriminations are simply more difficult to learn
impairment with repeated stimuli reflects deficits in memory and learning
different investigators finding differing results in similar tasks
intact perception despite MTL lesion
It could be that…
-> feature ambiguity tasks are more difficult to learn, BUT…
Buckley et al. (2001) show no relationship between task difficulty and impairment
Bussey et al. (2003) found normal learning of colour and size discrimination in monkeys, an equally difficult discrimination
-> impairment with repeated stimuli reflects deficits in memory and learning not perception
recent studies in humans and rats (Barense et al., 2007; Bartko et al., 2007) show impairments in oddity discrimination in high feature ambiguity conditions using trial unique stimulus presentation
-> different results, different studies
Stark & Squire (2000) = intact oddity discrimination with MTL lesions
Control subjects = 68.2% correct
MTL lesion subjects = 57.3% correct
Lee et al. (2005) = impaired oddity discrimination
Control subjects = 88.3% correct
MTL lesion subjects = 62.3% correct
However, in other cases performance of control subjects is well matched across studies
-> intact perception despite MTL lesion
If MTL = perception, there should be no intact perception with MTL lesions
This finding no more excludes perception linked with MTL lesions than does it exclude the role of MTL in learning and memory just because some memory tasks can be performed with MTL lesions eg. in Eacott study with small set sizes

10. Can We Strengthen the Argument for Perception?
use tasks that maximize perceptual demands
trial-unique stimuli
utilize converging evidence from behavioural neurophysiology
record from TE while perirhinal is inactivated
correctly characterize human lesions